Mass flow analysis of motor vehicle exhaust emissions is used for both diagnosing engine performance and for measuring the amounts of exhaust particulate. The use in diagnosing performance results from the discovery of measurable indicia in the engine exhaust gas which may be correlated with other engine test results to increase the accuracy of the diagnostic routines in isolating engine faults to specific subsystems. The use in particulate mass measurements is the result of mandatory emissions analysis imposed by the laws of many states. In diagnostic use the measuring apparatus may be used to detect or measure selected kinetic properties of the exhaust, i.e. turbulence, sub-cyclic frequency, whereas emissions analysis is concerned with the amounts of particulate.
One major concern in each case is the integrity of the sample. The exhaust gas sample must accurately reflect the dynamic characteristics and content of the exhaust gas, as emitted from the exhaust system of the vehicle. Since the exhaust flow is under positive pressure of the engine exhaust stroking the test apparatus must be interfaced with the vehicle exhaust system in sealed, positive pressure connection to eliminate dilution of the exhaust gas by the ambient air. Any such dilution affects both kinetic properties and mass flow density.
A second major concern is the ability to provide such a pressure seal connection under high volume motor vehicle testing conditions. This requires that the connection interface be capable of quick connection to and disconnection from a number of different make and model vehicles, such as in motor vehicle inspection (MVI) stations. The lack of standardization in the size and geometry of motor vehicle exhaust tail pipes poses a significant problem in providing such an interface. Although there are significant differences in geometry the results of statistical research indicates that at present approximately 86% of the motor vehicles in the Unites States have circular exhaust pipes. The diameter of the exhaust pipes vary significantly, ranging from 13/8 inches to 21/2 inches. The wide variation in tail pipe diameter presents a significant problem in providing a universal connection which can accommodate the nearly two-to-one variation.